Studies are focused on effects of renal pelvic peristalsis upon morphology, fluid compartments, and function of the renal medulla. Included are studies of animals in normal health as well as various disease models. Corollary studies deal with changes in renal morphology and function due to changes in protein and salt intake. The following specific aspects will be investigated: I. The intra- and extracellular fluid compartments in the renal papilla of the antidiuretic hamster change when the pelvic wall contracts during the peristaltic wave. Experiments will be carried out on: a0 diuretic hamsters, b) hamsters with inhibited pelvic peristalsis and c) hamsters in which the renal pelvic wall has been removed. Morphometric measurements and chemical analyses of the two renal papillae from each hamster will be carried out. II. Two experimental models (papillary necrosis and interstitial nephritis) will be studied in rats for changes in pelvic peristalsis, papillary water and solute content, and renal function. III. The interstitial cells of the papilla appear to form continuous channels throughout the inner medulla, which conduct interstitial fluid toward the outer medulla. Further studies will include mapping of these cells and tracing various electron-dense markers from the papillary interstitium to lymphatics at the border between medulla and cortex. IV. A significant change in the thin limbs of the loops of Henle has been found when rats are placed on a low protein high-salt diet. Studies are planned to determine a) if other morphological changes occur in the inner medulla, b) the effect of low protein diet without added salt and c) the effects of reinstituting normal protein intake on morphologic and physiologic parameters. The studies are of importance for an increased and more complete understanding of the mammalian concentrating mechanism in health and disease.